Use uncanonicalized StartOfWordPosition() in visible selection start adjuster

This patch hoists the creation of VisiblePosition from StartOfWord
to its caller ComputeStartRespectingGranularityAlgorithm to reduce
the usage of VisiblePosition.

Bug: 657237
Change-Id: If1489b363717d751f6e0d8ebc323f6e80045c8af
Reviewed-on: https://chromium-review.googlesource.com/c/1330760Reviewed-by: Yoichi Osato <yoichio@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607907}

third_party/blink/renderer/core/editing/selection_adjuster.cc

@@ -127,8 +127,9 @@ class GranularityAdjuster final {
         // kNextWordIfOnBoundary);
         const VisiblePositionTemplate<Strategy> visible_start =
             CreateVisiblePosition(passed_start);
-        return StartOfWord(visible_start, ChooseWordSide(visible_start))
-            .DeepEquivalent();
+        const PositionTemplate<Strategy> word_start = StartOfWordPosition(
+            passed_start.GetPosition(), ChooseWordSide(visible_start));
+        return CreateVisiblePosition(word_start).DeepEquivalent();
       }
       case TextGranularity::kSentence:
         return StartOfSentence(CreateVisiblePosition(passed_start))